The present invention is directed toward a method and apparatus for clearing a powder accumulation in a powder delivery tube, and more specifically, toward a method and apparatus for removing a build-up of coal on the interior wall of a coal lance that delivers powdered coal to a blast furnace blowpipe.
A typical blast furnace comprises an elongated section known as a stack, a hearth, and a section known as a bosh located between the hearth and the stack. A high temperature blast gas, usually compressed air, is pumped through blowpipes and though openings in the walls of the hearth called tuyeres into the upper portion of the hearth. Material known as the burden, which generally includes limestone, iron-bearing material, and a carbonaceous material such as coke, is intermittently charged into the furnace at the top of the stack and allowed to move down the stack. In the zone adjacent the tuyeres, the coke or other carbonaceous material burns, and the heat of combustion smelts the ore to produce molten pig iron. The products of combustion, being at relatively high temperature, serve to preheat the burden as they pass upwardly through the stack.
In a process known as xe2x80x9cfuel injection,xe2x80x9d a supplemental fuel such as coal is introduced into the furnace with the blast gas. This is typically done by inserting a coal lance through the wall of a blowpipe near a tuyere and blowing powdered coal from a source of coal through the coal lance. Fuel injection reduces the amount of coke needed to operate a blast furnace. Because coal is less expensive than coke, this process also increases the efficiency of iron production. Fuel injection and its benefits are discussed in more detail in U.S. Pat. No. 3,368,804, which patent is hereby incorporated by reference.
While fuel injection results in more efficient blast furnace operation, passing powdered coal through tubes that normally carry only a compressed gas can lead to problems. For example, it has been found that, over time, powdered coal tends to accumulate on the inner walls of the blowpipes and coal lances. If this coal is not removed promptly, the high temperatures may convert it to coke, a hard material that is very difficult to remove without damaging the wall to which it is attached. If a coal lance becomes blocked, no supplemental fuel will be injected into the furnace through a particular tuyere. If a blowpipe becomes blocked, no blast gas or coal will enter the furnace through the tuyere connected to that blowpipe. These blockages not only can damage the coal lances and blowpipes, but also affect combustion conditions near a particular tuyere and thus can also affect the smelting operation.
To clear such accumulations, many blast furnaces are provided with a purging system that pumps a fluid such as nitrogen through the coal lance and/or blowpipe periodically to dislodge accumulations of coal. However, with such systems, a balance must be struck between cycling the system too often, which wastes resources and can adversely affect furnace operation, and cycling the system too infrequently and thereby allowing accumulations to form. It is generally necessary to err toward purging too often to reduce the likelihood that blockages will occur.
Various systems are known from the prior art for monitoring conditions in a fuel injection system and for detecting and clearing accumulations of coal. For example, U.S. Pat. No. 3,368,804 shows a fuel injection control for a blast furnace that uses a magnetic flow transmitter to measure the flow of powdered coal through a coal delivery pipe. A purge valve 27 is opened to purge the fuel line and tuyere with water when the flow rate decreases, indicating an actual or imminent blockage. U.S. Pat. No. 4,519,587, incorporated herein by reference, shows another system for clearing a blockage when a pressure sensor in the coal delivery pipe indicates that the flow of coal is slowing due to an accumulation of coal downstream from the sensor. U.S. Pat. No. 5,397,108 shows a system for monitoring conditions in a blowpipe using an optical sensor. When the tuyere or blowpipe becomes blocked, less light from the fire in the furnace reaches the sensor and an alarm is sounded.
Prior art systems such as the above rely on sensors partially or entirely mounted in the fuel delivery system itself. These sensors are exposed to a hostile environment and must either be designed to withstand the conditions present or be adequately shielded therefrom. Thus the sensors tend to be expensive, and, as the coal delivery system or blowpipe must be modified to allow the use of such sensors, they are also expensive to install and maintain. Moreover, a typical blast furnace may have on the order of thirty-eight tuyeres, blowpipes and coal lances, and thus need thirty-eight separate sensors. It would therefore be advantageous to provide a method of monitoring the flow rate of coal through a coal lance that uses a relatively inexpensive sensor that does not need to be inserted into the fuel delivery system in order to detect conditions therein indicative of an imminent blockage.
These problems and others are addressed by the present invention which comprises a system for detecting and automatically clearing a blockage or accumulation of material in a powder delivery tube using a fluid under pressure which system detects the blockage or accumulation by monitoring the temperature or another condition on the outside of the powder delivery tube. While the present system could be used in a variety of environments where powder is delivered through a tube that needs to be purged when a blockage begins to form, it finds particular use in the environment of a coal delivery system in a blast furnace wherein coal is delivered from a source to a blowpipe. The invention will be described hereinafter in the environment of a blast furnace, it being understood that it would function equally well in other environments.
Powdered coal is ground in a process that heats the coal, and as this powdered coal flows through the coal delivery system, heat is also generated by friction between the coal particles and between the coal particles and the wall of the tube carrying the coal. This heat can be measured from the outside of the coal lance or coal pipe. More heat is generated the faster the coal moves through the tube. In the preferred embodiment, a thermocouple is attached to the outside of the coal delivery pipes, upstream from each coal lance, to measure the temperature of the outside walls of the coal pipes. A drop in temperature in one of the coal pipes is taken to indicate that the flow of coal within that coal pipe has slowed and that a blockage is beginning to form. The supply of coal to that coal lance is then stopped and a purge cycle is started to blow nitrogen though the coal lance and into the blowpipe to clear out the lance. The flow of coal is restarted after the purge cycle is complete.
The purge system preferably includes two valves in the coal tube and a purge line connected to the coal tube between these valves. The purge line also has a valve for connecting a source of fluid under pressure, nitrogen gas, for example, to the coal tube. All three valves are remotely controllable by a programmable logic controller (PLC) and/or a computer. The valves in the coal tube are normally open and the valve in the purge line is normally closed. However, when a blockage is detected, the valve upstream (toward the coal source) from the purge line is closed to temporarily stop the flow of coal, and the valve in the purge line is opened for a set period of time, three minutes for example, to clear any accumulations of coal. Optionally, after this portion of the purge cycle is complete, the valve downstream from the purge line may be closed and the upstream valve opened so that the purge fluid will flow toward the coal source and clear any accumulations of coal in that direction as well. When the entire purge cycle is complete, the valve in the purge line is closed and the two valves in the coal tube are returned to their open state to allow the flow of coal to resume.
A blast furnace operates under various ambient conditions which can affect the temperature of the coal tubes. For example, the temperature of all coal tubes may vary throughout the day or year as the local temperature rises or falls. Likewise, the temperatures of the various coal pipes around the furnace may be affected by wind so that the coal pipes on one side of the furnace are hotter than those on the other. To address these differences, a fixed xe2x80x9cnormalxe2x80x9d temperature is not assumed, but rather a floating average of the temperatures of a group of coal pipes is measured, and deviations from this average are noted. While the temperature of all coal pipes in a system could be averaged, in the preferred embodiment, the coal pipes are divided into two groups and a floating average temperature for each of these two groups is monitored. What is important is that deviations from a normal operating temperature, generally about 130 degrees F., are detected regardless of the absolute value of that normal temperature. When the temperature of a given coal pipe drops more than 10 degrees F. from normal, a purge cycle is initiated.
For a period of about five minutes after a purge, the temperature of the coal pipe may be lower than normal; therefore, no purges are carried out during this five minute period, even if the detected temperature is outside the normal range. Starting about five minutes after a purge, the system is once again set to execute purges when further temperature drops are detected. Preferably, an alarm is sounded at each purge to notify an operator of a potential problem so that the operator can determine whether further action is warranted, such as manually cleaning or replacing a badly blocked coal lance.
It is therefore a primary object of the present invention to provide a system for detecting and removing powder accumulations in a powder delivery system.
It is another object of the present invention to provide a system for detecting powder accumulations in a powder delivery system using sensors mounted outside the tubes carrying the powder being delivered.
It is a further object of the present invention to provide a system for detecting a partial blockage of a powder delivery tube by measuring the temperature of a surface of a powder delivery tube.
It is still another object of the present invention to provide a system for purging a coal tube in a blast furnace fuel injection system to remove accumulations of coal before the coal completely blocks the coal tube.
It is still a further object of the present invention to provide an apparatus for clearing accumulations of coal from individual coal lances of a blast furnace fuel injection system which apparatus includes thermocouples mounted on the outside surfaces of coal pipes connected between a source of coal and a plurality of coal lances for determining the temperature of each coal pipe and a controller for operating a purging system in response to detected temperature drops to purge a given coal lance before it becomes completely blocked.
In furtherance of these objectives, a purging system for clearing an accumulation of powder from a powder delivery tube is provided which includes a sensor mounted outside the powder delivery tube between a powder source and a powder destination for producing signals representing a flow rate of powder within the powder delivery tube. A source of fluid under pressure is selectively connected to a first location on the powder delivery tube through a first valve and a controller operatively connected to the sensor controls the first valve in response to the signals from the sensor.
A method of clearing an accumulation of coal in a coal tube connecting a source of coal to a tuyere is also provided comprising the steps of monitoring a condition on an exterior portion of the coal tube indicative of a flow rate of material through the coal tube and determining when the flow rate has dropped to below a predetermined level. When the flow rate drops, the flow of coal though the coal tube is stopped and the tube is purged with a fluid under pressure; then the flow of coal is restarted.
Another aspect of the invention comprises a method of clearing an accumulation of coal in a coal tube comprising the steps of monitoring the temperature of a coal tube and stopping the flow of coal though the coal tube when the temperature falls below a predetermined level and purging the coal tube with a fluid under pressure before restarting the flow of coal through the coal tube.
Another aspect of the invention comprises a system including a blast furnace having at least one tuyere, a blowpipe for delivering a heated fluid to the tuyere, a source of coal, and a coal tube connected between the source of coal and the blowpipe. A source of pressurized fluid is connected to the coal tube at a first location and a first valve is connected between the source of pressurized fluid and the coal tube for controlling the flow of fluid from the source of pressurized fluid. A second valve is located in the coal tube between the first location and the source of coal and a third valve is located in the coal tube between the first location and the tuyere. A controller is provided for controlling the first, second and third valves in response to signals from a temperature measuring device operatively connected to the controller and the coal tube.
A further aspect of the invention comprises a purging system for clearing an accumulation of coal from a coal lance fed by a coal pipe which system comprises a temperature sensor connected to the coal pipe for producing signals representative of a sensed temperature, a source of fluid selectively connectable to the coal lance through a valve, and a controller operatively connected to the sensor for controlling the valve in response to the signals from the sensor. The controller opens the valve for a period of time when the temperature of the coal pipe falls below a given level.
Another aspect of the invention comprises a method of regulating the flow of coal in a system for delivering coal from a source of coal to a coal lance comprising the steps of first connecting the source of coal to the coal lance with a coal pipe then providing a source of fluid under pressure and connecting the source of fluid to the coal pipe at a first location. A first valve is placed between the source of fluid and the first location and a second valve is placed between the source of coal and the first location while a third valve is placed between the first location and the coal lance. The first valve is normally closed while the second is normally open. The temperature of the coal pipe is measured, and, if the temperature of the coal pipe falls below a given value, the second valve is closed and the first valve is opened for a first period of time.